The present invention relates to directional control valves, and more particularly, to such valves which are both pressure-compensated and pilot-operated.
The present invention is especially suited for use with proportional flow control valves, and will be described in connection therewith. By "proportional", it is meant that changes in the output flow of fluid from the control valve to the motor which is being controlled are generally proportional to changes in the input, which may be a mechanical input movement or an electromagnetic input, etc.
As will be described in greater detail subsequently, the present invention may be utilized advantageously in a four-way, three- or four-position directional and flow control valve, or in a three-way, three-position directional and flow control valve. For simplicity of illustration, the invention will be described in connection with a three-position, three-way valve. In most commercial directional and flow control valves, various added features are considered desirable, or perhaps even necessary, for the valve to be functionally satisfactory, one example of such an added feature would be the provision of inlet check valves, so that a load under high pressure cannot cause a back-flow (or reverse flow) from the load back through the valve and out the inlet port.
Pilot-operated flow control valves of the type to which the present invention relates are known, generally, from U.S. Pat. Nos. 2,526,709 and 2,600,348. In such pilot-operated valves, there is a main valve spool which is capable of controlling both direction and quantity of fluid flow from an inlet port to a work port. The position of the main valve spool is determined by a pilot pressure which results from movement of a pilot spool disposed slidably within the main valve spool. Movement of the pilot spool communicates pilot pressure to the appropriate end of the main valve spool to move the main valve spool to the desired position. In such pilot-operated valves, the relationship of the main valve spool to the pilot spool is simply that of a "follow-up", i.e., subsequent to movement of the pilot spool, the main valve spool follows the pilot spool until the main valve spool is again in a "neutral" position relative to the pilot spool. Typically, the only factor which determines the position of the main valve spool is the position of the pilot spool.
A typical pressure-compensated directional flow control valve is illustrated and described in U.S. Pat. No. 3,602,243, assigned to the assignee of the present invention and incorporated herein by reference. In such valves, there is typically a main valve spool, normally manually actuated, and a separate pressure-compensating valve section, the function of which is to regulate the flow from the inlet to the main valve spool to maintain a constant pressure differential across the main valve spool, regardless of the rate of fluid flow through the main valve spool. The pressure compensating valve typically includes a pressure-compensating spool which is positioned in response to the differential between inlet pressure and the pressure downstream of the main valve spool.
The addition of pressure compensation capability to a typical directional flow control valve adds substantially to the complexity of the valve section, requiring several additional "cores" in the valve housing casting, and a substantial amount of additional machining of the bore in which the pressure compensating spool is disposed. In addition, the pressure compensating spool itself, and any associated biasing springs, etc., represent a further added manufacturing cost.
If a particular directional flow control valve, whether pilot-operated, or pressure-compensated, is to be used in connection with a load sensing system, it is typically necessary to include within the system a load sensing priority flow control valve. The function of such a valve is to direct the appropriate amount of flow to a priority load circuit, while directing the remainder of the flow to an auxiliary load circuit. As is well known to those skilled in the art, typical load sensing priority flow control valves also add substantially to the cost and complexity of a typical hydraulic circuit.